Orbital-free density functional theory applied to NaAlH4

We present the application of orbital-free density functional theory (OF-DFT) to NaAlH(4), a potential hydrogen storage material, and related systems. Although the simple Al and NaH structures are reproduced reasonably well by OF-DFT, the approach fails for the more complex NaAlH(4) structure. Calculations on AlH(3) show that the failure to describe the Al-H interaction is related to the kinetic energy functionals used rather than the local pseudopotentials which are required within the OF-DFT approach. Thus, systems such as NaAlH(4) present a challenge which awaits the development of more reliable orbital-free kinetic energy functionals.     

Comparative study of the interaction of O2 and C2H4 with small vanadium clusters from density functional theory

We have studied C(2)H(4) and O(2) molecules separately or simultaneously for adsorption on V(n) (n = 2-8) clusters, and V(n) clusters catalyzed ethylene oxidation to acetaldehyde using spin-polarized density functional theory calculations. Molecular adsorption and clear size-dependent adsorption energy are predicted for C(2)H(4). O(2) is dissociately adsorbed with nearly constant adsorption energy. In the case of coadsorption, O(2) and C(2)H(4) adsorb on the V(n) surface simultaneously. Each keeps the same adsorption form, molecular or dissociative, as in separate adsorption. The noted cooperative effect is noted in C(2)H(4) and O(2) coadsorption, which activates the C-C double bond of C(2)H(4) and favors its oxidization. Furthermore, both the separate and coadsorptions result in magnetic enhancement or reduction of V(n), which is found to be dependent on the cluster size and the adsorbates. In addition, we reveal the reaction mechanism of V(2) (V(6))-catalyzed ethylene oxidation to acetaldehyde and find the overall reaction is exothermic and barrierless.     

Studies of iridium nanoparticles using density functional theory calculations

The energetics and the electronic and magnetic properties of iridium nanoparticles in the range of 2-64 atoms were investigated using density functional theory calculations. A variety of different geometric configurations were studied, including planar, three-dimensional, nanowire, and single-walled nanotube. The binding energy per atom increases with size and dimensionality from 2.53 eV/atom for the iridium dimer to 6.09 eV/atom for the 64-atom cluster. The most stable geometry is planar until four atoms are reached and three-dimensional thereafter. The simple cubic structure is the most stable geometric building block until a strikingly large 48-atom cluster, when the most stable geometry transitions to face-centered cubic, as found in the bulk metal. The strong preference for cubic structure among small clusters demonstrates their rigidity. This result indicates that iridium nanoparticles intrinsically do not favor the coalescence process. Nanowires formed from linear atomic chains of up to 4-atom rings were studied, and the wires formed from 4-atom rings were extremely stable. Single-walled nanotubes were also studied. These nanotubes were formed by stacking 5- and 6-atom rings to form a tube. The ring stacking with each atom directly above the previous atom is more stable than if the alternate rings are rotated.     

The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory

We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)(2)P(2)O(7), VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J(OPO) = -156.8 K and J(O) = -68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J(layer) = 19.2 K and J(chain) = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J(OPO) originates from a super-exchange interaction through the bridging -O-P-O- unit. In contrast, J(O) results from a direct overlap of 3d(x(2)-y(2)) orbitals on two vanadium atoms in the same V(2)O(8) motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO)(2)-V are covalent and rigid, whereas the bonds of V-(O)(2)-V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J(O), changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J(O) to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O(2) can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.     

Hybrid density functional theory band structure engineering in hematite

We present a hybrid density functional theory (DFT) study of doping effects in α-Fe(2)O(3), hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure α-Fe(2)O(3) crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.     

Insights into the Schrock 'chop-chop' reaction gained from density functional theory and preparation and structure of W2(mu-PhCCPh)(SC6H4-2-Me)6

Computations employing density functional theory on the reactions between ethyne and the model compounds (HE)3M identical to M(EH)3, where M = Mo and W and E = O and S, predict that the alkyne adducts M2(mu-C2H2)(EH)6 are thermodynamically favored with respect to the metathesis products HC identical to M(EH)3 except when M = W and E = O; the reaction between (tBuO)3 W identical to CPh and 2-MeC6H4SH (> 3 equiv.) yields W2(mu-PhCCPh)(SC6H4-2-Me)6 consistent with expectations based on the calculations.     

The local structure of protonated water from x-ray absorption and density functional theory

We present a combined x-ray absorption spectroscopy/computational study of water in hydrochloric acid (HCl) solutions of varying concentration to address the structure and bonding of excess protons and their effect on the hydrogen bonding network in liquid water. Intensity variations and energy shifts indicate changes in the hydrogen bonding structure in water as well as the local structure of the protonated complex as a function of the concentration of protons. In particular, in highly acidic solutions we find a dominance of the Eigen form, H(3)O(+), while the proton is less localized to a specific water under less acidic conditions.     

Equilibrium adsorption in cylindrical mesopores: a modified Broekhoff and de Boer theory versus density functional theory

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.     

Magnetizability tensors from auxiliary density functional theory

The working equations for the calculation of the magnetizability tensor in the framework of auxiliary density functional theory with gauge including atomic orbitals (ADFT-GIAO) are derived. Unlike in the corresponding conventional density functional theory implementations the numerical integration of the GIAOs is avoided in ADFT-GIAO. Our validation shows that this simplification has no effect on the accuracy of the methodology. As a result, a reliable and efficient implementation for the calculation of magnetizabilities of systems with more than 1000 atoms and 14 000 basis functions is presented.     

Spin-multiplet energies from time-dependent density functional theory

Starting from a formally exact density-functional representation of the frequency-dependent linear density response and exploiting the fact that the latter has poles at the true excitation energies, we develop a density-functional method for the calculation of excitation energies. Simple additive corrections to the Kohn-Sham single-particle transition energies are derived whose actual computation only requires the ordinary static Kohn-Sham orbitals and the corresponding eigenvalues. Numerical results are presented for spin-singlet and triplet energies. © 1996 John Wiley & Sons, Inc.     

Continuum states from time-dependent density functional theory

Linear response time-dependent density functional theory is used to study low-lying electronic continuum states of targets that can bind an extra electron. Exact formulas to extract scattering amplitudes from the susceptibility are derived in one dimension. A single-pole approximation for scattering phase shifts in three dimensions is shown to be more accurate than static exchange for singlet electron-He(+) scattering.     

Applying density functional theory on tautomerism in 3,4-dihydropyrimidin-2(1H)-ones

In the present study, the density functional theory (DFT) and Gibbs free energy calculations were performed to investigate the stability and tautomerism of C4-substituted-3,4-dihydropyrimidin-2(1H)-ones. Three different forms are possible for the ethyl 3,4-dihydropyrimidinones (ethyl 4-aryl-6-methyl-3,4-dihydropyrimidin-2(1H)-one-5-carboxylates, ethyl 4-aryl-2-hydroxy-6-methyl-1,4-dihydropyrimidine-5-carboxylates and ethyl 4-aryl-2-hydroxy-6-methyl-3,4-dihydropyrimidine-5-carboxylates) forms that the most stable form is ethyl 4-aryl-6-methyl-3,4-dihydropyrimidin-2 (1H)-one-5-carboxylates (keto-form). The obtained data showed that the substitution on the C4-substitut position can be effective on the equilibrium constant (K _eq).     

Characterization and optical properties of mechanochemically synthesized molybdenum-doped rutile nanoparticles and their electronic structure studies by density functional theory

The optical and electronic properties of molybdenum (Mo) doped rutile TiO₂ prepared by the mechanochemical method were studied both experimentally and using density functional theory (DFT). The synthesized nanoparticles were characterized by XRD, TEM, EDS-MAP, and XPS. The XRD results showed the successful incorporation of Mo in the rutile crystal lattice. High-resolution TEM images illustrated a decreasing trend in the (110) d-spacing for samples doped up to 3 at%. The shift toward higher binding energies in the XPS spectra was due to the higher oxidization tendencies of Mo⁵⁺ and Mo⁶⁺ substituted in Ti⁴⁺ sites. The optical behavior of samples was examined by UV–Vis and photoluminescence spectroscopy. The bandgap energy value of rutile was reduced from 3.0 eV to 2.4 eV by 2 at% Mo doping. The DFT calculations showed a reduction of bandgap energy value of rutile to 2.35 eV with 2 at% Mo, which is in harmony with the experimental results. The creation of energy states below the conduction band because of Mo doping was identified as the reason for reducing the bandgap energy and photoluminescence emission of rutile.     

The structure of the second-order reduced density matrix in density matrix functional theory and its construction from formal criteria

Some formal requirements for the second-order reduced density matrix are discussed in the context of density matrix functional theory. They serve as a basis for the ad hoc construction of the second-order reduced density matrix in terms of the first-order reduced density matrix and lead to implicit functionals where the occupation numbers of the natural orbitals are obtained as diagonal elements of an idempotent matrix the elements of which represent the variational parameters to be optimized. The numerical results obtained from a first realization of such an implicit density matrix functional give excellent agreement with the results of full configuration interaction calculations for four-electron systems like LiH and Be. Results for H2O taken as an example for a somewhat larger molecule are numerically less satisfactory but still give reasonable occupation numbers of the natural orbitals and indicate the capability of density matrix functional theory to cope with static electron correlation.     

Excitation energies from range-separated time-dependent density and density matrix functional theory

Time-dependent density functional theory (TD-DFT) in the adiabatic formulation exhibits known failures when applied to predicting excitation energies. One of them is the lack of the doubly excited configurations. On the other hand, the time-dependent theory based on a one-electron reduced density matrix functional (time-dependent density matrix functional theory, TD-DMFT) has proven accurate in determining single and double excitations of H(2) molecule if the exact functional is employed in the adiabatic approximation. We propose a new approach for computing excited state energies that relies on functionals of electron density and one-electron reduced density matrix, where the latter is applied in the long-range region of electron-electron interactions. A similar approach has been recently successfully employed in predicting ground state potential energy curves of diatomic molecules even in the dissociation limit, where static correlation effects are dominating. In the paper, a time-dependent functional theory based on the range-separation of electronic interaction operator is rigorously formulated. To turn the approach into a practical scheme the adiabatic approximation is proposed for the short- and long-range components of the coupling matrix present in the linear response equations. In the end, the problem of finding excitation energies is turned into an eigenproblem for a symmetric matrix. Assignment of obtained excitations is discussed and it is shown how to identify double excitations from the analysis of approximate transition density matrix elements. The proposed method used with the short-range local density approximation (srLDA) and the long-range Buijse-Baerends density matrix functional (lrBB) is applied to H(2) molecule (at equilibrium geometry and in the dissociation limit) and to Be atom. The method accounts for double excitations in the investigated systems but, unfortunately, the accuracy of some of them is poor. The quality of the other excitations is in general much better than that offered by TD-DFT-LDA or TD-DMFT-BB approximations if the range-separation parameter is properly chosen. The latter remains an open problem.     

Atomistic and electronic structure of bimetallic cobalt/rhenium clusters from density functional theory calculations

We have carried out computational density functional investigations of Co I Re J (J=0,1,2; I+J=14) metal atom clusters. Through thorough optimization of geometry, spin polarization, and electronic configuration, the most stable structures for each cluster have been identified. While the global minima are found to be well defined and energetically well separated from other local minima, the study reveals a plethora of different structures and symmetries only moderately higher in energy. A key point of interest is the effect of doping the cobalt clusters with rhenium. Aside from significant structural reorganizations, rhenium is found to stabilize the clusters and couple down the spin. Furthermore, the most stable clusters comprise highly coordinated rhenium and, in the case of Co 12 Re 2, Re-Re bonding. Our results are compared to earlier experimental and computational data.     

Towards a structure-performance relationship for hydrogen storage in Ti-doped NaAlH4 nanoparticles

Hydrogen storage properties of Ti-doped nanosized (~20 nm) NaAlH(4) supported on carbon nanofibers were affected by the stage at which Ti was introduced. When Ti was deposited first followed by NaAlH(4), sorption properties were superior to the case where NaAlH(4) was deposited first followed by NaAlH(4). This was the result of both a smaller NaAlH(4) particle size and the more extensive catalytic action of Ti in the former material.     

NMR shielding tensors from auxiliary density functional theory

The working equations for the calculation of NMR shielding tensors in the framework of auxiliary density functional theory are derived. It is shown that in this approach the numerical integration over gauge-including atomic orbitals can be avoided without the loss of accuracy. New integral recurrence relations for the required analytic electric-field-type integrals are derived. The computational performance of the resulting formalism permits shielding tensor calculations of systems with more than 1000 atoms and 15,000 basis functions.